Construction machine with working attachment

Abstract

A construction machine, including: a hydraulic pump; a working attachment including a boom, a boom cylinder, and another fluid-supplied cylinder closer to a leading end than the boom cylinder; first and second operation devices for the boom cylinder and the fluid-supplied cylinder; first and second control valves; a flow-rate-limiting section; and an anti-cavitation circuit. The fluid-supplied cylinder receives fluid-supply from the hydraulic pump, together with the boom cylinder. A reaction force upon work accompanying extension of the fluid-supplied cylinder increases the rod-side pressure of the boom cylinder and reduces a head-side pressure. The flow-rate-limiting section limits a supply flow rate to a head-side fluid chamber of the boom cylinder only during a boom-raising-operation with the rod-side pressure higher than the head-side pressure in the boom cylinder. The anti-cavitation circuit replenishes hydraulic fluid from a tank to the head-side fluid chamber.

The invention claimed is: 
     
       1. A construction machine, comprising:
 a lower traveling body; 
 an upper slewing body installed on the lower traveling body so as to be slewable; 
 a tank which stores hydraulic fluid; 
 a hydraulic pump which discharges hydraulic fluid from the tank; 
 a working attachment attached to the upper slewing body and having a boom, an arm attached to a leading end of the boom, a working device attached to a leading end of the arm, a boom cylinder which actuates the boom, an arm cylinder which actuates the arm, and a working cylinder which actuates the working device, wherein at least one cylinder of the arm cylinder and the working cylinder is a fluid-supply-target cylinder which, together with the boom cylinder, receives a supply of hydraulic fluid from the hydraulic pump, the fluid-supply-target cylinder being positioned closer to a leading end of the working attachment than the boom cylinder, the attachment being arranged so as to receive a reaction force during work involving extension of the fluid-supply-target cylinder, the reaction force acting in a direction of raising the boom to thereby increase a rod-side pressure of the boom cylinder and reduce a head-side pressure thereof; 
 a first operation device which is operated to move the boom cylinder; 
 a second operation device which is operated to move the fluid-supply-target cylinder; 
 a first control valve which operates in response to the operation of the first operation device to control a motion of the boom cylinder by supplying hydraulic fluid to the head-side fluid chamber of the boom cylinder when an operation for raising the boom is performed; 
 a second control valve which operates in response to the operation of the second operation device to control a motion of the fluid-supply-target cylinder; 
 a flow-rate-limiting section which imparts a limitation to a supply flow rate from the hydraulic pump to the head-side fluid chamber of the boom cylinder only when an operation for raising the boom is performed in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof, and imparts no limitation to the supply flow rate in a situation where the rod-side pressure of the boom cylinder is equal to the head-side pressure thereof or lower than the head-side pressure thereof; and 
 an anti-cavitation circuit including a conduit directly interconnecting the head-side fluid chamber and the tank so as to bypass the first control valve to replenish the head-side fluid chamber of the boom cylinder with hydraulic fluid from the tank through the conduit bypassing the first control valve, and a single check valve provided at a mid-portion of the conduit to prevent hydraulic fluid from inflow into the tank. 
 
     
     
       2. The construction machine according to  claim 1 , wherein the fluid-supply-target cylinder is the arm cylinder, and the flow-rate-limiting section limits the supply flow rate from the hydraulic pump to the head-side fluid chamber of the boom cylinder, during a compound operation of simultaneously performing an arm retracting operation for extending the arm cylinder and a boom raising operation for relieving the rod-side pressure of the boom cylinder caused by a reaction force upon work in the arm retracting operation. 
     
     
       3. The construction machine according to  claim 1 , further comprising a regeneration circuit for supplying return fluid from a rod-side fluid chamber of the boom cylinder to the head-side fluid chamber of the boom cylinder, during the boom raising operation in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof. 
     
     
       4. The construction machine according to  claim 3 , further comprising: respective detectors which detect the head-side pressure and rod-side pressure of the boom cylinder, a discharge pressure of the hydraulic pump, and an operation amount of the first operation device; and
 a flow-rate-control device that varies the flow rate to be limited in the flow-rate-limiting section, 
 wherein the flow-rate-control device performs: 
 calculating a flow rate of hydraulic fluid which can be regenerated from the rod-side fluid chamber to the head-side fluid chamber of the boom cylinder during the boom raising operation in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof; 
 calculating a required flow rate of hydraulic fluid to be supplied to the head-side fluid chamber in accordance with an area ratio of the boom cylinder, on the basis of the flow rate of hydraulic fluid flowing out of the rod-side fluid chamber of the boom cylinder; 
 calculating a required pump flow rate of hydraulic fluid to be supplied from the hydraulic pump to the head-side fluid chamber of the boom cylinder in order to prevent cavitation; 
 calculating a flow rate, or a parameter corresponding thereto, for the flow-rate-limiting section, out of the required pump flow rate; and 
 operating the flow-rate-limiting section so as to obtain a flow rate corresponding to the calculated flow rate or the calculated parameter. 
 
     
     
       5. The construction machine according to  claim 1 , wherein the flow-rate-limiting section is in a conduit connecting the hydraulic pump to the head-side fluid chamber of the boom cylinder. 
     
     
       6. A construction machine, comprising:
 a lower traveling body; 
 an upper slewing body installed on the lower traveling body so as to be slewable; 
 a tank which stores hydraulic fluid; 
 a hydraulic pump which discharges hydraulic fluid from the tank; 
 a working attachment attached to the upper slewing body and having a boom, an arm attached to a leading end of the boom, a working device attached to a leading end of the arm, a boom cylinder which actuates the boom, an arm cylinder which actuates the arm, and a working cylinder which actuates the working device, wherein at least one cylinder of the arm cylinder and the working cylinder is a fluid-supply-target cylinder which, together with the boom cylinder, receives a supply of hydraulic fluid from the hydraulic pump, the fluid-supply-target cylinder being positioned closer to a leading end of the working attachment than the boom cylinder, the attachment being arranged so as to receive a reaction force during work involving extension of the fluid-supply-target cylinder, the reaction force acting in a direction of raising the boom to thereby increase a rod-side pressure of the boom cylinder and reduce a head-side pressure thereof; 
 a first operation device which is operated to move the boom cylinder; 
 a second operation device which is operated to move the fluid-supply-target cylinder; 
 a first control valve which operates in response to the operation of the first operation device to control a motion of the boom cylinder; 
 a second control valve which operates in response to the operation of the second operation device to control a motion of the fluid-supply-target cylinder; 
 a flow-rate-limiting section which imparts a limitation to a supply flow rate from the hydraulic pump to a head-side fluid chamber of the boom cylinder, only when an operation for raising the boom is performed in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof, and 
 an anti-cavitation circuit for replenishing the head-side fluid chamber of the boom cylinder with hydraulic fluid from the tank, wherein: 
 each of the first operation device and the second operation device comprises a remote control valve having an operation member and adapted to output a pilot pressure corresponding to an operation of the operation member; 
 the first control valve and the second control valve comprise respective hydraulic-pilot-operated selector valves which are operated in accordance with respective pilot pressures outputted by the remote control valves which make up the first operation device and the second operation device, respectively; 
 the flow-rate-limiting section includes a hydraulic-pilot-operated flow-rate-control valve provided in a head-side supply conduit interconnecting the hydraulic pump and the head-side fluid chamber of the boom cylinder, the hydraulic-pilot-operated flow-rate-control valve having a fully-open position of maximum opening area and a throttle position of smaller opening area than the opening area of the fully-open position; 
 the flow-rate-control valve has a first pilot port into which the rod-side pressure of the boom cylinder and the pilot pressure for boom raising outputted by the remote control valve that makes up the first operation device are introduced and a second pilot port into which the head-side pressure of the boom cylinder is introduced; and 
 the flow-rate-control valve is configured to be switched to the throttle position by a difference of respective pressures introduced at both of the pilot ports, only when an operation of raising the boom is performed in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof. 
 
     
     
       7. The construction machine according to  claim 6 , further comprising a regeneration circuit for supplying return fluid from a rod-side fluid chamber of the boom cylinder to the head-side fluid chamber of the boom cylinder, during the boom raising operation in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof. 
     
     
       8. The construction machine according to  claim 7 , further comprising:
 respective detectors which detect the head-side pressure and rod-side pressure of the boom cylinder, a discharge pressure of the hydraulic pump, and an operation amount of the first operation device; and 
 a flow-rate-control device that varies the flow rate to be limited in the flow-rate-limiting section, wherein the flow-rate-control device performs: 
 calculating a flow rate of hydraulic fluid which can be regenerated from the rod-side fluid chamber to the head-side fluid chamber of the boom cylinder during the boom raising operation in a situation where the rod-side pressure of the boom cylinder is higher than the head-side pressure thereof; 
 calculating a required flow rate of hydraulic fluid to be supplied to the head-side fluid chamber in accordance with an area ratio of the boom cylinder, on the basis of the flow rate of hydraulic fluid flowing out of the rod-side fluid chamber of the boom cylinder; 
 calculating a required pump flow rate of hydraulic fluid to be supplied from the hydraulic pump to the head-side fluid chamber of the boom cylinder in order to prevent cavitation; 
 calculating a flow rate, or a parameter corresponding thereto, for the flow-rate-limiting section, out of the required pump flow rate; and 
 operating the flowrate-limiting section so as to obtain a flow rate corresponding to the calculated flow rate or the calculated parameter.